Ask the pilot

What's that Great Dane doing beneath my Airbus? And other questions only the pilot can answer.


Patrick Smith
January 20, 2006 3:30PM (UTC)

If I understand it right, Ask the Pilot's hardcore readers tend to be split among two main groups -- a sort of right brain/left brain fan base. On the left are those who appreciate the more straightforward pieces -- accident analysis, airline evaluations, technical and procedural insights. On the right are those favoring the more impressionistic ones -- the personal reminiscences, travel anecdotes and quirky cockpit stories.

Which are more enjoyable to write? It depends, though I lean toward the latter. Forced to pick a favorite-ever article, I'd probably go with this one.

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And after all, this is Salon and not an aviation magazine. My target audience has never been those predisposed to flying machines. Thus, of the letters I receive, most flattering are the ones from people only peripherally, if at all, interested in airplanes or the airlines, but who find the column strangely addictive.

But my overarching mission is to blend both camps: letting aerophiles know that aviation reportage needn't be dry and impersonal, while turning on the average person to the weird, interlinked drama of flight and travel. Maintaining that crossover at times means walking a tightrope, and audience alienation is a constant danger: "That was a very nice piece; elegant and lyrical," is followed immediately by, "Your column sucked; I wish you'd do an article on the advantages of trans-ionic frequency modulation in aircraft radios."

Now and then, it's best to ignore one side altogether and aim solely for the other. This week, it's a rootsy, back-to-basics questions-and-answers session.

I frequently ride aboard Airbus planes. During taxi, either prior to takeoff or after landing, it's common to hear a loud whirring sound emanating from beneath the floorboards. It's a robotic "WHOOF, WHOOF, WHOOF" -- like the noise a very angry and deranged Great Dane might make.

This is a very common query, and pertains to later-generation, twin-engine Airbus models -- namely the A320 series (includes the sub-variants A318, A319 and A321) and the much larger A330. Almost every frequent flier has encountered this sound at one time or another. Crews rarely make efforts to explain it, leaving passengers befuddled, and sometimes worried. Because the noise is akin to a motor repeatedly trying -- and failing -- to start, there's often the assumption that something is malfunctioning.

What you hear is a device called the power transfer unit, or PTU, which is designed to ensure adequate hydraulic pressures during single-engine operations. To conserve fuel, it's fairly routine for two-engine planes to taxi with an engine shut down. Each engine normally pressurizes its own hydraulic system, but with a motor not running, that leaves one system without a power source. That's where the PTU comes in, helping left power the right, or right power the left. Since it is activated only when the pressure falls below a certain level, the PTU cycles on and off, on and off. It also does a self-test when the right engine is started, so you'd hear it then as well, regardless of hydraulic pressures.

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That's the short answer, which ought to appease the typical nervous or curious rider. For gearheads, or those wishing to impress (or bore) their seatmates, here's a description by correspondent Dave English, Airbus A320 pilot and aviation writer:

"The A320 has two main hydraulic systems that operate flaps, landing gear, flight controls, cargo doors and brakes. These are called Green and Yellow. This has nothing to do with the color of the hydraulic fluids; they're just useful labels. The Green system is powered by the number 1 (left) engine-driven pump. The Yellow system is powered by the number 2 (right) engine-driven pump. Should we lose or depower an engine-driven pump, there is a neat device called PTU -- a two-way reversible motor pump that allows the Green system to pressurize Yellow, or Yellow to pressurize Green. When you taxi with only one engine running, the PTU kicks in to power the opposite side. There is no fluid transfer, keeping each system otherwise independent."

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English points out that the PTU racket is not audible from the cockpit. "But," he admits, "it's annoying as heck in the main cabin." It's anybody's guess why Airbus didn't come up with quieter machinery, and most non-Airbus aircraft employ a different layout altogether.

In the United States, the largest operators of A320s and/or A330s are JetBlue, Northwest, U.S. Airways and United. To reduce PTU noise, some airlines ask crews to run a separate electric pump during single-engine taxi.

Not long after takeoff from Geneva, our captain announced, in typical nonchalant fashion, "We are now level at 35,000 feet." I thought nothing of this and gazed lazily out the window. Then, something struck me. Directly below were the Alps -- a great, undulating mountain scape of towering peaks, deep valleys, and wide plateaus. How could the plane have been "level at 35,000 feet," when obviously our height above the ground was constantly changing?

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Because "altitude" and height above the ground aren't necessarily the same thing. Usually, they are not. This is a good, basic question that surprisingly no reader has ever asked before. You'd expect altitude to be a pretty straightforward concept, seeing how the pilot's foremost mission is to avoid crashing into things, but it's not so simple.

A plane's altimeters (typically there are three independent units) -- and, in turn, all air traffic control instructions -- are referenced to sea level, not to the ground. Parked at the gate at Mexico City, they will read about 7,500 feet. Thus, when the captain announces that you're cruising at 35,000 feet, the surface of el mundo may or may not be 35,000 feet below you. Over the ocean, yes, but over a high plateau, mesa or mountain range, no. A sea level datum allows aircraft to be safely and evenly sequenced. You couldn't have a sector full of planes, all at 15,000 feet, if 15,000 kept changing each time you passed above a hill or depression.

Altimeters have no idea where the ground is, strictly speaking. Essentially barometers, they are set by dialing or programming in the local barometric pressure, gauged in inches of mercury (or metric millibars) like the ones given out by TV meteorologists. The atmospheric standard for sea level is 29.92 inches (1013.2 mb), but actual pressures will vary from place to place, depending on the weather. If you enjoy tuning in to United's Channel Nine, you'll hear crews being advised of the local value. "Altimeter: two-niner eight-six," for example.

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En route, above certain heights -- 18,000 feet in the U.S., but usually lower elsewhere -- all aircraft change over to the single standard of 29.92 inches. At this point, cruising altitudes become known as "flight levels." Thirty-five thousand "feet" is technically "flight level three-five-zero."

There's a separate cockpit unit called a radio altimeter that measures a plane's position relative to the turf itself. Height above ground level, or AGL, is referenced when flying close to the surface -- up to maybe 1,500 feet or so-- such as during instrument approaches. Over uneven terrain, the radio altimeters, unlike the main altimeters, will be constantly changing.

When the air got rough on a trip from Honolulu to the West Coast, the pilot said: "We cannot climb any higher until we burn more fuel." I fail to understand the connection between altitude and weight. We had even more fuel on the ground, and we took off just fine. Why can't we keep climbing?

This is similar to why flights are sometimes weight restricted from high-elevation airports. The higher you climb, the thinner the atmosphere, reducing aerodynamic efficiency and output of the engines. A plane might perform just fine from Honolulu, which sits at sea level, but it will not behave with the same gusto at 35,000 feet. A flight may have to "step climb" its way to the optimum altitude, ascending in stages as fuel is consumed. This is another reason pilots think of fuel in terms of pounds, not gallons.

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While that same atmospheric thinness rewards you with markedly better fuel efficiency once you get there, it also makes for a certain aerodynamic precariousness. A plane's ceiling is predicated not only on the physical ability to reach an altitude, but the need to maintain stall margins.

I've been disappointed with your fixation on the media's alarmist attitude toward airplane mishaps. The shortcomings of the American media are no longer news to me. What I want to hear dissected is why such a pervasive climate of fear surrounds the industry to begin with -- one the papers and networks never hesitate to capitalize on. Why do so many people, despite the numbers, see air travel as treacherous?

There's a clear chicken-or-the-egg aspect to this, but the questioner is on to something. Perhaps if people weren't so nervous about flying to begin with, mass media wouldn't have the leverage it does to continuously terrify them. That nervousness is there for two main reasons. One is excusable, the other not so much.

First and foremost, millions of people find it hard to reconcile with the notion of traveling hundreds of miles per hour, far above the earth, inside pressurized tubes weighing hundreds of thousands of pounds. Flying is not natural for human beings, and while it doesn't quite violate the laws of physics, it does seem to violate any and all common sense. Technology has made it work, but while airplane travel isn't statistically dangerous, inherently it's another story. I wouldn't call us ignorant, exactly; most Americans can't tell you how a television set works either, never mind a 747. The difference is, a TV set can't crash into a skyscraper and kill 3,000 people -- one of them you. In the end, everybody from veteran pilots to first-time fliers contemplates their mortality when stepping onto a plane, as well they should.

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Second, the airlines themselves shoulder a heavy portion of the blame for being such lousy communicators. Airlines have a terrible habit of responding to anomalies -- be it a minor malfunction or a catastrophic crash -- in one of two ways: either with total silence or, perhaps worse, by employing hideous oversimplifications. A flight from Las Vegas is canceled because "it's too hot to fly"; a crew aborts a landing because "a plane crossed in front of us." At Flagstaff, Ariz., counter staff at America West Express told a group of delayed passengers that several volunteers were needed to give up their seats. When passengers asked why, they were told, "We need to lighten the load. The plane has been having problems and we're afraid one of the engines might cut out." The result is nearly total lack of trust from the public. People dislike airlines and don't believe anything they say -- partly because they never actually say anything -- or, when they do, it's both condescending and terrifying.

However, neither innate human fears nor an airline's nurturing of them (more on the latter next week) is a good enough excuse for the treacly infotainment peddled by the major networks, or the shoddy reportage that shows up in newspapers. When things go wrong, the media don't have terribly much to work with, it's true. Then again, the media's job is not to perpetuate myths and half-truths. Its job is to expose them and to seek the whole truth.

After today, barring any major occurrences, my media critiques are hereby placed on indefinite hiatus. But in a closing salvo, I bring you the following from Barrie Clement of the U.K. newspaper the Independent.

"Number of deaths in plane crashes doubles," sings the headline. "The number of people killed in air crashes doubled last year to a seven-year high, according to the latest figures. The toll of fatal accidents rose by six to 34, but the number of passengers and crew who perished leapt from 466 to 1,050."

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And so on. As a 2005 retrospective, the story is a kind of yin to last week's Ask the Pilot yang, in which I stressed that the year just ended was actually one of our safest. To describe deaths as having "leapt" from 466 to 1,050 is at best disingenuous. A single disaster can double or triple a year's death tolls, with little relevance to the larger picture. (My own crash totals were slightly different than Clement's, but tallies will vary source to source, depending on whether cargo, air taxi and other peripheral operators are included.)

And at no point does Clement remind us that while 1,050 deaths represents a seven-year high, worldwide traffic has grown by an astounding 25 percent in that same seven-year stretch -- an increase of half a billion annual passengers. According to preliminary figures from ICAO, 2005 marked the first year in history in which boardings reached 2 billion people, up from just under 1.9 billion in 2004. (Don't get me wrong -- I'm not one of those people who believe that unbridled eco-industrial growth is a good and healthy thing for planet Earth. But when talking about safety, the prodigious expansion of civil aviation is unavoidable context.)

Using Clement's and ICAO's data, here's what that "leap" parses out to: In 2004, just under 1.9 billion people flew, and 466 were killed. That's .000000245 percent of all passengers. For 2005, it was 2 billion fliers and 1,050 casualties, or .000000525 percent.

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CONTEST ANSWERS:

Shockingly, it took a day and a half before somebody solved Ask the Pilot's Jan. 13 aero-trivia quiz. That's either a testament to my skill at finally outmaneuvering Google, or lack of a decent incentive. Normally a correct submission is on hand by morning of publication day. Robert Holland of Decatur, Ga., got his '80s out and was the winner:

1. Of history's 10 worst air disasters, six took place in the 1980s. What were they? (For background, click here.

  • Japan Air Lines 747 crashes near Mt. Fuji killing 520 ('85)
  • Bomb destroys an Air India flight over the Atlantic killing 329 ('85)
  • All 301 aboard a Saudia L-1011 die in a flash fire at Riyadh ('80)
  • Iran Air jet is downed by the U.S. Navy destroyer Vincennes killing 290 ('88)
  • The bombing of Pan Am 103 over Scotland kills 270 ('88)
  • Korean Air Lines 747 is shot down by a Soviet fighter killing 269 ('83)

    2. In 1981, the Iran hostages saga came to an end when the 52 remaining U.S. captives were released. On which commercial airline, and which type of aircraft, were they winged to freedom?

  • An Air Algerie (the national carrier of Algeria) Boeing 727

    3. In 1985, a gripping two-week drama ensued after a TWA Boeing 727 was hijacked on a flight between Athens and Rome. Which electro-industrial dance band later wrote a song based on this hijacking?

  • Nitzer Ebb (the song is called "TWA")

    4. Only two commercial airliners were ever built with four aft-mounted engines -- two on either side of the tail. One of these appears on the cover of an album from 1982. Name the aircraft, the group and the album.

  • Vickers VC-10, on the English Beat's "Special Beat Service"

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    Do you have questions for Salon's aviation expert? Send them to AskThePilot and look for answers in a future column.


  • Patrick Smith

    Patrick Smith is an airline pilot.

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